The practice of thickening liquids has become one of the most frequently used interventions for swallowing impairment (dysphagia). However, the terminology used to describe thickened liquids (such as nectar-thick and honey-thick) is subjective and we lack empirical evidence about how alterations in liquid consistency affect swallowing function and physiology. The International Dysphagia Diet Standardisation Initiative is a multidisciplinary task force (www.iddsi.org) that has recently developed a new taxonomy of terms to label different levels of liquid consistency used in dysphagia management; these are paired with operational definitions and practical gravity-flow measurement techniques that can be used by caregivers and clinicians to confirm the category of any liquid at the time of preparation or serving. The goal of the current proposal is to measure the in vitro physiological flow of liquids representative of the IDDSI levels of liquid consistency (thin, slightly-thick, milly-thick, moderately-thick and extremely-thick). Physiological measures of liquid flow require an understanding both of the rheological properties of the liquid (gravity-flow; viscosity) and the forces that are applied to the liquid during swallowing (tongue pressure; swallowing muscle contraction). Additional sensory attributes of the bolus (such as slipperiness, graininess and cohesiveness) are also likely to be relevant. We will measure these properties and study bolus flow in healthy adults to establish a reference perspective of expected flow in the context of healthy tongue pressure generation. We will then collect comparative measures in individuals with dysphagia of different etiologies (stroke, acquired brain injury, oropharyngeal cancer, post cervical spine surgery, and neurodegenerative disease) to determine how alterations in swallowing motor function impact liquid flow. These measurements will provide information to guide clinicians in determining optimal levels of thickening to recommend for patients with dysphagia. This research is highly significant because it will establish a new foundation of understanding with respect to the influence of thickened liquids on swallowing. This is essential for advancing clinical practice and setting the stage for future treatment efficacy research.